Sure, I'm not sure if the link will work, but
here I'll try to attach a link related to some £9.50 singles on Sunday.
Obviously, it won't always be £9.50 due to rush hour, and whatnot, but for personal experience, I've always been able to find them on Fridays and Sundays when I've needed to travel for the weekend.
As for Stratford International, I don't think so unfortunately. You can get some tickets where you go from Stratford to Ashford, then change platforms to go from Ashford to Canterbury West, but these take more time, and in my experience, can cost like £20 more - so it's not really worth it personally.
I hope this helps
If you have any more questions, please feel free to ask!
Best,
David
University of Kent Representative
Edit: I've done some checking, and found some tickets from St Pancras Intenational to Canterbury West
here, which ranges from ~£10 -£13, however, some have changes, but others don't! So, that's good news for you! Just make sure that you properly make sure to plan before leaving, so as to not miss, have to change platforms, or overspend (as I learnt the hard way
, oh well)